Don't know about April but Azamara has a wonderful itinerary Hong Kong to Singapore stopping at both Hanoi and Saigon enroute Singapore. We did almost identical itinerary on Regent Seven Seas Mariner in 2005 and loved it.

In December of 2012 my wife and I will be sailing on Celebrity Millennium. It is an excellent itinerary from Singapore to Hong Kong with overnight port stays in/near Ho Chi Minh City and Hanoi. It is also reasonably priced.

Take care,
Mike

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"There is a great difference between being well traveled and just having been to many places." ~Me